The present invention relates to an improved deterministic bus access method which is appropriate where a number of stations (or nodes) on a bus which wish to communicate, are located at a distance for example more than 5 meters apart from each other. In particular, but not exclusively, the invention relates to the field of Local Area Networks (LANs).
Any system which allows one or more transmitting stations to communicate with one or more receiving stations over a shared serial or parallel bus using some form of (statistical) time division multiplexing (TDM), requires an access mechanism (or protocol) to determine which one of several active transmitting stations is to be allowed access to the shared bus. In systems where the stations are located close to each other (e.g. less than 5 meters apart) some centralised arbitration system can De used e.g. a central controller. However, in more physically distributed systems the electrical signal propagation delay can limit the effectiveness (speed) of the access mechanism.
Where the access mechanism has to be supported by a bus, constraints imposed on the speed of the access mechanism by the basic nature of a bus can be particularly severe (especially with short packet transmission times and physically long buses). These constraints are firstly that any signal transmitted on a bus is broadcast so that it is received by all stations connected to it and secondly, that the maximum time between a station sending a signal and another station receiving the signal is the end-to-end propagation time of the bus.
The performance of one widely used method of bus access, Collision Sense Multiple Access/Collision Detect (CSMA/CD) is limited mainly by the second consideration; because here the lower limit of the contention slot time which defines the basis of the CSMA/CD back-off algorithims, is fixed by the fact that twice the end-to-end propagation time of the bus has to elapse after start of transmission before a station can be sure that it is the only station transmitting over the bus.
The performance of another widely used method of access, token passing, is limited chiefly by the first consideration because here the token has to be directed to one node at a time by the addition of adequate address information (to the token). The time to transmit the token from one node to the next mode (in the logical ring) plus some allowance for internal node processing and node to node propagation times generally yields fairly lengthy node to node polling times (e.g. of approximately 50 uS in the case of a typical 5 Mbps carrier band implementation of the IEEE 802.4 token passing bus).
The time for the token to circulate completely round the logical ring of such a network is therefore the node to node polling time multiplied by the number of network nodes: for a typical 30 node network this results in a token circulation time of about 1.5 ms (30.times.50 uS). This circulation time results in excessively long waiting time delays for short packets of data.
Less widely used determinsitic methods of bus access are bit map protocols such as (Multi Level Multi Access (MLMA). In each of these cases the total time taken by the determinsitic methods is based on the fact that the duration of each bit of information in the access method must equal or exceed the end-to-end propagation time of the bus. (e.g. 5 uS for a 1 Km bus) so that all stations are aware of when each particular bit in the access protocol has been set.
It will be appreciated that the efficiency of an access protocol is limited by the ratio of packet transmission time to access time. This is of particular importance as increased data rates result in decreased transmission times for the same length of packet.